Ionic compounds generally exist in the form of crystals composed of positively charged cations and negatively charged anions which pull electrostatically on each other. Such ionic compounds dissolve in water and various other liquids to form liquids that conduct electricity, i.e., electrolyte solutions.
Some ionic compounds maintain a liquid state at room temperature and do not solidify even at very low temperatures. Such ionic compounds which maintain a liquid state at room temperature or below are referred to in particular as “room-temperature fused salts” or “ionic liquids.” To minimize electrostatic interactions between the cations and anions which make up the ionic liquid, either the cations or anions or both are molecular ions of a significant size. Moreover, to minimize the charge and electrostatic interactions, either or both are monovalent.
Active research efforts are being made to employ such ionic liquids as electrolytes in batteries and other applications. However, in general, ionic liquids have a high hydroscopic property and are difficult to handle in air. As a result, their use has remained limited.
In light of these circumstances, the 1-ethyl-3-methylimidazolium tetrafluoroborate reported by Wilkes et al. in 1992 is a remarkable ionic liquid that can be handled even in air. This new ionic liquid led to the synthesis of many ionic liquids which are combinations of numerous alkylimidazolium cations having different side chains with various anions.
Such developments have gradually led to efforts to use ionic liquids as electrolytes in nonaqueous electrolyte secondary cells. For example, JP-A 8-245828 (Patent Reference 1), JP-A 10-265673 (Patent Reference 2) and JP-A 10-265674 (Patent Reference 3) disclose solid electrolytes which use polymeric compound complexes containing a room-temperature fused salt (ionic liquid) and a polymeric compound (and lithium salt). Solid electrolytes which employ such polymeric compound complexes are able to reduce the tendency for leakage associated with the use of liquid electrolytes.
Each of the foregoing patent references mentions the use of substances such as cyclic amidine onium salts, pyridine onium salts and aliphatic quaternary ammonium salts of organic carboxylic acids as room-temperature fused salts. However, because these room-temperature fused salts do not have very broad potential windows, they are readily subject to reductive decomposition during charging and discharging of the electrochemical devices such as secondary cells and tend to degrade, thus lacking a performance adequate for practical use.
Also, these room-temperature fused salts have relatively high solidification points, and thus remain inadequate in terms of increasing the low-temperature properties of electrochemical devices such as secondary cells.
Moreover, in the above-described electrolytes which employ a polymeric compound complex, because the polymer itself either lacks ionic conductivity or has only a poor ionic conductivity, the polymer complex obtained with the polymer has a greatly diminished ionic conductivity.
In light of these circumstances, one object of the present invention is to provide nonaqueous electrolytes which contain an ionic liquid and a polymeric compound and afford electrical double-layer capacitors and nonaqueous electrolyte secondary cells having excellent charge-discharge efficiency, stability and low-temperature characteristics. Additional objects of the invention are to provide electrical double-layer capacitors and nonaqueous electrolyte secondary cells in which such nonaqueous electrolytes are used.